Begin by locating the water supply shutoff valve, typically on the wall behind the toilet. Turn the valve clockwise until it stops to cut off the water supply to the tank. Flush the toilet to drain the majority of the water from the tank. There will be about an inch of water left at the bottom; use a large sponge to soak up this remaining water, wringing it out into a bucket until the inside of the tank is completely dry. Place old towels on the floor around the base of the toilet to catch any residual drips.

Position a small bucket or towels directly under the point where the water supply line connects to the bottom of the toilet tank's fill valve. Using an adjustable wrench, grip the coupling nut on the supply line. Turn the nut counter-clockwise to loosen and disconnect it from the tank. A small amount of water may drip out, which the bucket or towels will catch.

Locate the two or three tank-to-bowl bolts that secure the tank. The nuts are on the underside of the toilet bowl's rear ledge. Use a deep-well socket or an adjustable wrench to hold the nut steady underneath while using a long screwdriver (usually a flathead) to turn the bolt head from inside the tank. Once the nuts are removed, carefully lift the tank straight up off the bowl. Be prepared for its weight and awkwardness. Set the tank down gently on the prepared towels or a piece of cardboard, placing it upside down or on its back.

With the tank removed, you will see the large, spongy tank-to-bowl gasket. Pull off the old gasket from the threaded outlet of the flush valve. Remove the old tank bolts and their rubber washers from inside the tank. Use a rag to thoroughly clean the area around the flush valve outlet on the bottom of the tank and the water inlet hole on the top of the bowl. Any grime or sediment can prevent a good seal. Install the new tank-to-bowl gasket by stretching it over the flush valve outlet. Install the new bolts by pushing them through the holes from inside the tank, ensuring the new rubber washers are on the inside to create a seal.

Carefully lift the tank and position it back onto the bowl, meticulously guiding the new bolts through their corresponding holes in the bowl ledge. Once seated, press down gently on the tank to help compress the new gasket. Working from underneath, place the new metal washers and nuts onto the bolts. Hand-tighten the nuts first. Then, using your wrench, tighten the nuts incrementally, alternating between them to ensure even pressure. Tighten until the tank is stable and does not wobble, but do not crank them down with excessive force.

Reconnect the water supply line to the fill valve on the bottom of the tank. Hand-tighten the coupling nut, then use an adjustable wrench to give it a final quarter-turn to snug it up. Slowly turn the water supply valve counter-clockwise to turn the water back on. Listen and watch as the tank fills. Use dry paper towels to wipe around the new gasket and bolt nuts to check for any signs of moisture. Flush the toilet several times to put pressure on the new gasket. Continue to check for leaks. If a small drip appears from a bolt, tighten the corresponding nut another quarter-turn until it stops.